Unprocessed hydrocarbons derived from crude oil and natural gas tend to contain a number of sulfur compounds. These compounds or the sulfur they contain must be removed before the hydrocarbons are suitable for use. There are two prime reasons for this. First, the release of sulfur or sulfur compounds into the atmosphere is a significant environmental concern which is coming under increased regulatory constraints. Second, sulfur must be at a very low concentration in various hydrocarbon streams in order to meet the quality standards for these hydrocarbon products. It is therefore necessary for petroleum refineries, gas processing plants and other industrial facilities to remove sulfur compounds of various types from raw material fractions and from partially processed hydrocarbon fractions such as hydrogen-rich gas streams, natural gas, diesel fuel, naphtha and LPG. This need to remove the sulfur compounds is independent of whether the hydrocarbons are intended for use as a motor fuel or as a petrochemical feedstock.
Many different techniques such as hydrotreating are widely used for removing sulfur compounds from hydrocarbon streams. Most of these techniques include a step in which a liquid phase or gas phase hydrocarbon stream is contacted with an aqueous stream containing a dissolved alkaline material. The aqueous stream, commonly referred to as "caustic" is employed to remove acidic sulfur compounds from both vapor phase or liquid phase process streams. The result is the formation of a sulfur-rich alkaline aqueous stream; that is, one containing an increased concentration of a sulfur compound such as hydrogen sulfide. The rich aqueous stream must be regenerated so that it can be reused. The subject invention provides a process for regenerating hydrogen sulfide rich alkaline aqueous streams in a manner which converts the hydrogen sulfide to recovered elemental sulfur. To accomplish this, the subject invention passes the hydrogen sulfide rich aqueous stream into a biological reaction zone, a term which is used herein to describe both single reactor and multiple reactor apparatus. It may contain zones for performing both aerobic and anaerobic reactions. This biological reaction zone employs naturally occurring microorganisms that catalyze the conversion of sulfur compounds to elemental sulfur which is physically removed. The subject invention then purifies the treated aqueous stream to remove additional elemental sulfur.